This invention relates to screen printing apparatus and specifically, to an improved device for controlling and adjusting the spring rate associated with the screen holding arms usually associated with such apparatus.
Printing machines in the screen printing industry typically print small cap images and larger T-shirt images. In most cases, printers use wood or aluminum frames of various sizes, typically a 9".times.12" for caps and a 20".times.22" for T-shirts and the like. In the case of wood, typically a soft wood such as pine, which has the advantage of being light in weight, is employed in the frame construction, with individual frame sides generally about 1 inch thick and about 11/2 inches wide.
There are instances, however, where oversize frames are required for even larger images such as for pant legs, sleeves, decorative banners, etc., and in those cases where more than one image is to be provided on a single screen. In such cases, the oversize frames are significantly heavier than the largest of the normally used frames, i.e., the 20".times.22" frame. In fact, it is known to use 2.times.4's constructed from Kiln-dried hard wood in the construction of oversize frames. These larger frames have proven to be difficult and even unworkable, when used with conventional screen printing machines primarily due to their increased weight.
In a typical screen printing machine, a screen (or screen frame) holding arm is mounted for pivotal movement between a printing, or down, position to a non-printing, or up, position relative to a platen upon which rests the item to be printed. The screen holding arm is generally provided with a clamping or holding mechanism which grips a screen of the type described hereinabove. Most such machines employ coil extension springs extending between the machine head and a point intermediate the ends of the screen holding arm, to control movement of the screen and screen holding arm from the printing to the non-printing position and vice versa. Of course, these springs are designed to extend only to certain length, beyond which the spring is subject to permanent damage from plastic deformation. The springs currently in use in the screen printing industry are sufficient to handle the normal range of frame sizes up to the 20".times.22" size, and even slightly larger. However, for substantially larger and heavier frames, these springs are not sufficient as explained below.
In conventional screen printing machines, using conventional screen sizes, the inner point of attachment of the spring, i.e., at the machine head, is generally slightly below the pivot point of the screen holding arm. Typically, when the screen is raised to a non-printing position (generally about 60.degree. from horizontal) and released, it will remain in the raised position. At the same time, when the arm is lowered to a printing position, the arm will remain lowered, with the screen in contact with the item to be printed, so that the operator can use both hands to print the cap, T-shirt or other items.
When oversize frames are utilized in such machines, the weight of the frame overcomes the spring tension so that the frame will not stay in the up, or non-printing position. Thus, the operator must support the screen in the raised position to prevent it from falling to the lower, or printing position. This, of course, is an undesirable and even unworkable situation.
If larger, stiffer springs are used to accommodate these oversize frames, the overall flexibility of the machine is reduced because the operator cannot thereafter switch back to the smaller, lighter cap or T-shirt frames. This is because the larger, stiffer springs will keep the lighter frames in a normally biased upward position so that an operator would not be able to release the frame after lowering it into a printing position. It is, of course, important that the operator be able to free his hands to print the image when the frame is lowered. Thus, in the past, moving between extreme frame sizes thus involved a time consuming change of springs as well. In addition, the stiffer larger springs employed for oversize frames are typically made of hardened steel, and create a substantial risk of injury in the event of breakage.
Other approaches have been taken when utilizing the larger frames for controlling the movement of the screen holding arms. For example, compression rather than extension springs have been tried, as have pressurized compression gas cylinders. However, compression springs have usually not proven to be satisfactory and, in the case of gas cylinders, there typically is no ability to adjust the cylinders to change the rate at which the connecting (piston) rod extends or retracts within the cylinder.
Other manufacturers have employed turnbuckles attached to the end of the extension springs to regulate the tension, or extension, of the spring. It has also been attempted to employ four springs, rather than two, for each screen holding arm. Neither of these techniques has met with any significant degree of acceptance in the trade.
In commonly assigned U.S. Pat. No. 4,907,506, improvements are disclosed which relate to an ability to adjust not only the spring rate, but also to adjust the degree of leverage to facilitate movement of the screen holding arm between the printing and non-printing positions by adjusting the points of attachment of the springs to the machine frame.
In this present invention, another approach is taken to provide a degree of adjustability for spring rates in otherwise conventional screen holding arms. Specifically, a secondary spring is provided as an add-on for use with each of the primary coil springs associated with conventional screen holding arms. In other words, for each pair of coil springs utilized in conjunction with a screen holding arm, a secondary spring is operatively associated with a respective one of the coil springs to provide an additional degree of adjustability to the spring rate of the primary springs. In one exemplary embodiment, these "internal" or secondary springs may also be coil springs attached, for example, to the same attachment points as the primary springs, and which extend, for example, through the center of the primary coil springs.
It is contemplated that a number of different secondary spring pairs, each with a different spring rate, can be selectively chosen to provide the desired overall spring rate and hence the desired movement of the screen holding arm.
In another exemplary embodiment, still further adjustability may be provided by attaching the secondary or internal springs to turnbuckle-type attachments so that the spring rate of each secondary spring pair can be altered in situ, i.e., without having to change the internal springs themselves.
In still another exemplary embodiment, adjustment of spring rate can be effected by attaching the secondary springs, at different times, directly to different coils of the primary springs. In other words, the larger the number of primary coils encompassed by the secondary spring, the greater will be the effect of the secondary spring on the primary spring rate.
In its broadest aspects, therefore, the present invention provides a screen printing machine including a head and at least one screen frame holding arm pivotally mounted on the head for movement about an axis toward and away from a platen to a lowered printing and raised non-printing positions, respectively, and including primary spring means associated with the at least one arm, the primary spring means attached at one end to first attachment means located on at least one arm and at another end to second attachment means located on the head for controlling the movement of at least one arm; the improvement comprising secondary spring means for adjusting the spring rate of the primary spring means.
The invention also relates to a method for adjusting the spring rate of a pair of primary springs attached at first ends to first attachment locations on a screen frame holding arm of a screen printing machine, and at second ends to second attachment locations on a machine head of the screen printing machine comprising the steps of:
a) providing a plurality of secondary spring pairs, having different spring rates;
b) selecting one of the secondary spring pairs as a function of the spring rates; and
c) mounting the selected one of the secondary spring pairs in operative association with the pair of primary springs.
Additional object and advantages of the subject invention will become apparent with the detailed description of the invention which follows.